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Leptogenesis and Bi-Unitary Parametrization of Neutrino Yukawa Matrix

We analyze the neutrino Yukawa matrix by considering three constraints: the out-of-equilibrium condition of lepton number violating process responsible for leptogenesis, the upper bound of branching ratio of lepton flavor violating decay, and the prediction of large mixing angles using the see-saw mechanism. In a certain parametrization with bi-unitary transformation, it is shown that the structure which satisfies the constraints can be characterized by only seven types of Yukawa matrices. The constraint of the branching ratio of LFV turns out as a redundant one after applying other two constraints. We propose that this parametrization can be the framework in which the CP asymmetry of lepton number violating process can be predicted in terms of observable neutrino parameters at low energy, if necessary, under assumptions following from a theory with additional symmetries. There is an appealing model of neutrino Yukawa matrix considering the CP asymmetry for leptogenesis and the theoretical motivation to reduce the number of free parameters.